This is what successful use of the tactics skill should tell you. You need to ask to make that check. Experienced combatants do that when they enter the room, not once the fight breaks out. That doesn't mean that the ref assumes the character doesn't do it until the player says it (often players forget) instead once combat is in the offing or actually starts then player gets that roll and the referee describes the situation in sufficient detail depending on the effect number.Some thoughts:
Of course D&D style combat habits are a problem. People who have those habits have expectations of how a combat is going to play out. D&D operates on quite a few flawed assumptions, and people accustomed to it don't do particularly well when playing a game operating of different assumptions. When this happens, they can either figure out what happened and try again, or they can get all mad and raise that mournful cry: "this is stupid, let's play D&D".
Something else is a limitation inherent to all ttrps: the disconnect between what the referee/GM imagines to be in the characters' environment, what has been defined in that environment, what should reasonably be in the environment, and the players' perception of what is in the characters' environment. If the characters are in some environment and a gunfight breaks out, their players don't have the immediate visual knowledge of what cover and concealment there is. In the excitement of the moment, people default to there isn't any and say "my guy does this" instead of "wait, what kind of cover and concealment is there". Stopping to ask that question is an unwelcome pause in the action.
Solution? The ref needs to do it.
The ref could have two descriptions of an environment, the usual description and the tactical description.
The usual description: it's a bar with ornate wood paneling, and so on and so forth.
The tactical description: the bar looks like it's made from local hardwood, the tables are too, but the chairs have wicker backs. The dinner service cart is made of relatively thin metal. The piano can conceal one or two guys. There are two sturdy hardwood columns flanking the main entrance.
When the combat is joined, the ref could state the tactical description, then move into the combat. This lets the players know what's there that is tactically useful, and also reminds them about cover, concealment, and using the environment tactically. Soon the players will come to expect this, and they'll get into the habit of thinking about cover, concealment, and other tactical options.
If the player wants to pre-empt the fight and position themselves advantageously they need to evaluate the potential threats and position accordingly by actively requesting a tactics roll. If they fail the roll and misread the situation they might be in a less advantageous disposition, but those are the breaks. It is no worse than finding it out at random once the fight kicks off.
As Traveller combat can be so deadly it is not too much of a burden to allow a little time to review the situation and preposition your team in decent positions. Once players have done it a few times, they will get in the habit or they will decide to just cut to the chase. Real combatants get caught out because they get overconfident or careless as 100% vigilance gets exhausting.