[I'm writing this from a hotel room in Leshan, China, as part of a 10-day 7-city self-guided tour, which may help explain my relative lack of participation in this discussion.]

Nesov, if by UDT you mean the version I gave in the article that AlpheNeil linked to in this post (which for clarity I prefer to call UDT1), it was intended that the agent knows its own source code. It doesn't explicitly look for copies of itself in the environment, but is supposed to implicitly handle other copies of itself (or predictions of itself, or generally, other agents/objects that are logically related to itself in some way). The way it does so apparently has problems that I don't know how to solve at this point, but it was never intended that locations of the agent are explicitly provided to the agent.

I may have failed to convey this because whenever I write out a world program for UDT1, I always use "S" to represent the agent, but S is supposed stand for the actual source code of the agent (i.e., a concrete implementation of UDT1), not a special symbol that means "a copy of the agent". And S is supposed to know its own source code via a quining-type trick.

(I'm hoping this is enough to get you and others to re-read the original post in a new light and understand what I was trying to get at. If not, I'll try to clarify more at a later time.)