Ravenous Bugblatter Beast of Traal
Sure. The players can have default actions that they do, either by explicitly telling the DM or by having it clearly established previously. That's not even table rules, that's just communication through words or deeds what the character would be doing.Table rules.
At my tables, adventurers that are not someplace safe and secure are assumed to be on the lookout. They are assumed to be looking for wandering monsters, trip wires, etc and not to be doing "nothing" and resting.
My table has found that continuously stating what to us is obvious, "While Mary is deactivating the trap, I am on guard" is simple boring, meaningless and un-fun. To us, it's not obvious that while Zippo is solving a puzzle everyone else is simply doing nothing. No, to us they are on the lookout for threats and dangers.
But each table gets to make it's own assumptions. Maybe DM's should address this OOC or at Session 0 if they don't know their player base. I admit is annoys me as a player when a GM assumes my PC is sitting around with their thumb us their tail pipe while another character is defeating the security system to the highly defended BBEG's place and says "You feel a tap on your shoulder, a guard asks you what your friend is doing."
However, that is not something for the DM to assume without that direction of the players. The DM never plays the characters. (They can control them through magic, but that is not playing them.)
If Wazo the Wise is a tired old man who takes every opportunity to sit and rest that comes up, it's not in the DM's perview to say "sorry, Wazo was alert for wandering monsters and not resting". Just as it's not the player's place to try to retcon that Wazo was doing anything but resting if they didn't say anything.
And absent of any defined default, it is not the DM's place to play the character by making assumptions they are doing things. And yes, I'm sure there are corner cases around automatic reactions like "when thrown into the water you didn't say you were trying to not drown". But even if a character screams when falling down a deep pit should be up to the player.