I’m a native English speaker here and it’s clear to me. I also think it’s somewhat tricky to express it more succinctly without changing the sentence a fair bit.

Essentially it means “it must have been the case that, at the moment we’re speaking of, someone was watching her.”

Your sentence is slightly different, in that it implies that, in general, someone was watching her during the timeframe we’re talking about, but not necessarily in the exact instant we’re talking about. Also, like the example above, your version could mean she’s someone who in general is watched (like the person who generally lives in Berlin), as opposed to someone being watched right during the instant we’re talking about.

Complicated!