It might seem strange, but I discarded a non suji 7 over a non suji 1. My read was that when shimocha discarded 5p, he likely had no 5p in his hand and I would discard 67p and hope to use 14p as a pair, thinking that they were dangerous, as the 5p discard could be 35->23 or 235->23(+unknown). Discarding 7p still gives about the same draws to get to tenpai, 1456p in exchange for 5678p. If I got into a tenpai that required me to discard 14p or 45s, I wouldn't know what to do for sure. I got to realize during the game that I made a good read, as they drew 1p and won (but, I ended up last regardless...

I believe 556p -> 56 and 455 -> 45 were both entirely possible but I guessed 23p was more likely than 45 or 56. But, just because I was right here doesn't mean my guess was right.
At that point, my read from safe to dangerous was:
2p - Almost always safe
367p - Would bet on being safe for holding on to a hand
45s - Unknown but dora middle tiles, wouldn't discard without them being almost guaranteed safe.
14p - Dangerous
I think a quote I heard from a mahjong video was something like "Rather than knowing what to discard, mahjong is about knowing what not to discard," and I think that's true for both efficiency and defense. When trying to discard efficiently, there's not necessarily a single correct discard, but there can be several poor discards of tiles that are valuable to your hand, and when trying to defend there's not necessarily a single safest discard, but there can be plenty of extremely dangerous discards. I think the key to playing strong in the midgame (neither too riskily nor too defensively) is having a good sense of the high danger discards and avoiding those, but continuing on other tiles even though they aren't clearly safe. The trick is that it can be very confusing. The example above is against a riichi, but shows how sometimes a non suji 7 can be safer than a 1. There's a lot to it and I can't explain it all, but here are some tips:
I would feel fine discarding suji of a recently discarded tile the majority of the game, especially if hands haven't changed much since the discard (people aren't discarding tiles out of their hand). I would feel a little safer discarding half suji (i.e. 5 when 8 is discarded), moreso if 234 of the suit are in players' discards. A big exception is if someone discards a tile next to the suji that was discarded recently. For example, if a 5 is discarded, then someone discards a 6 from their hand, I would be wary of discarding 5/8. Another exception is unusual missing tiles at the very end of a round. If, say, 6 is walled and three 8 are discarded but you can't see any 7, it becomes a bit dangerous, but this only factors in much in the last discards. Finally, you might play into hands still (I recently played into a big damaten iipeikou hand like this), but that's rare.
The early game discards (first 5-6ish) can tell you who to worry about. If a player is discarding tiles besides honors or terminals, and it doesn't look like honitsu/chanta, and the player plays pretty solidly, you might expect their hand to be pretty good, and you'll want to pay attention to them. Of course, people sometimes go from discarding honor tiles to instantly in tenpai, but it's less common.
You can play more aggressively when dora aren't in play (discarded honor tiles, you can see most/all of them).
Holding on to safe tiles for later in the game doesn't mean you won't play in to a hand, because you still have to decide when to play safe. Having a hand full of dangerous tiles doesn't mean you will play in to a hand either.
Pay attention to the suji in your hand. If you have multiple of a suji, it's more likely someone else needs it to make their hand. For example, if you have 44 77, then if you don't see 5 or 6 discarded you should worry about discarding either 4 or 7. In some cases, like, if you have 33667 and are in tenpai, with no other information discarding 7 may be safer, as you only have 1 of the 4/7 suji, but 4 of the 3/6 suji, so it's more likely someone needs 3/6 and has 4/7 in their hand. I feel like this applies even more when the tiles aren't in your hand, but are discarded by other players. If 114 are on the table and late in the game you have 234 and draw 1, if there's nothing to make 1/4 look safe (2/3 discarded in mid game or walled) it's probably more dangerous than usual. But still, in many cases it's fine to discard a 1 anyway, and avoiding discarding a 1 for a reason like this all the time is probably too defensive. This also goes against the idea of discarding a tile you have a lot of when you don't know what's safe, as it says it's more likely someone else doesn't have it, and so they need it.
In the late game (after 10-12ish discards) if a yakuhai hasn't been discarded it's probably dangerous, and even if you have 2, if it gets near the end of the game and you don't see one discarded it becomes more likely someone else has 2 as well.
The edges of the tiles being discarded by players and by the table in general are often the most dangerous. If you see 123 and 89 of a suit being discarded, you should be wary of 4/7 mid to late game. If you see 12, 89 being discarded, 3/6 and 4/7 are worrisome.
Against combined dangerous suji, like someone discards 4 from their hand and 2/5 and 3/6 look dangerous, if one of those suji is shown to be safe, the other one becomes even more dangerous. Against unknown riichi hands, if you see, say











I wouldn't worry too much about watching discards from hands unless you're worried about a player specifically for some reason.
I'm sure there's more, but this is all I can think of for now. Does anyone have any other thoughts on the topic?