... we might simply be picking up the spill from a beam powering something like a solar sail.

There are plenty of studies (by Geoff Landis in particular) of theoretical mechanisms which concentrate stellar light on a distant craft to achieve interstellar propulsion on a relatively inexpensive sustainable basis (once the original, suitably astronomical, technological nut has been paid for); so one might reasonably expect that it an intelligence universe such synchronised beams between the stars might be commonplace... & detectable, if they happen to spill into our sky.

There's a corollory to this, of course; & that is that the easiest beam to detect would be one pointed towards our system....

Robert