This topic has come up here many times before, including in this thread:
'Do undercover cops offer outcalls?'
I think the consensus of opinion is that female cops posing as hookers don't often do outcalls because of the danger stemming from not being able to control the environment like cops can when they book the room. A common m.o. for cops running a sting is for the girl to be near the bed and cops with guns drawn burst out of the bathroom. Obviously, you can't do that on an outcall. Also, if a UC did outcalls, there'd be no assurance the john's playpen would be located in the city where the UC works.