The third bullet point in the description of the Alert feat was updated in the PHB Errata. The description originally read:
Always on the lookout for danger, you gain the following benefits:
You can’t be surprised while you are conscious.
You gain a +5 bonus to initiative.
Other creatures don't gain advantage on attack rolls against you as a result of being hidden from you.
The third bullet point was changed to read:
- Other creatures don’t gain advantage on attack rolls against you as a result of being unseen by you.
As such, the benefit is not limited only to hidden opponents, but invisible ones as well (or those that you can not see for any reason).
However, the Alert feat does not grant you the ability to know where hidden enemies are or anything of that sort; it simply grants the mechanical benefit to you of them not getting advantage on their attack against you from being unseen, and prevents you from being surprised (which would prevent you from taking any actions or reactions until your turn during the first round of combat is over) while conscious.
It's not clear how enemies would get in range of you and then hide without you noticing if you can't be surprised.
Even if the enemies did manage to hide before you noticed, if your turn in the initiative order came before the enemies' turns, you could simply ready an attack against the first enemy to appear, or flee, or get behind cover and hide yourself.