Wouldn't the mass change cause some kind of detectable change in the satellite's trajectory over time?
(To be clear, I'd imagine they're only doing the inspection thing right now, but I do suspect they're at least tinkering with on-orbit capture, refueling, disabling etc. with an arm.)
Sure, but if You are lucky or smart enough, to use it close enough.
For example Soviet satellite-interceptors spent few years and made few attempts (each time with bigger explosive), before achieve enough cloud density .