Some monsters have "Telepathy" listed among their languages. The description of telepathy states (MM, p. 9):
Telepathy is a magical ability that allows a monster to communicate
mentally with another creature within a specified range. The contacted
creature doesn't need to share a language with the monster to
communicate in this way with it, but it must be able to understand at
least one language. A creature without telepathy can receive and
respond to telepathic messages but can't initiate or terminate a
telepathic conversation.A telepathic monster doesn't need to see a contacted creature and can
end the telepathic contact at any time. The contact is broken as soon
as the two creatures are no longer within range of each other or if
the telepathic monster contacts a different creature within range. A
telepathic monster can initiate or terminate a telepathic conversation
without using an action, but while the monster is incapacitated, it
can't initiate telepathic contact, and any current contact is
terminated.
Does this ability allow them to sense the position of all creatures within the radius of their telepathy, given that "A telepathic monster doesn't need to see a contacted creature"?
Best Answer
No, telepathy doesn't inherently grant knowledge of creatures
You don't get any abilities that are not listed in the description. So if it does not say that you can sense creatures, then you can't.
Think of it like yelling in a dark room. I don't have to see you, know where you are, or even whether you are in the room in order for me to attempt to be heard by you. Where you are and whether you're there only affects your ability to hear me, not my ability to yell.
An example of what you might see if a creature had both telepathy and such a detection ability (as suggested by guildsbounty in a comment) is the Intellect Devourer (p. 191 of the Monster Manual). It has telepathy (to a radius of 60 feet), and also has the separate ability to detect sentient minds (within 300 feet) using its Detect Sentience trait.