After having Nick Valentine as a companion for a part of the main storyline I switched back to Piper, however I see no option to send him back to his agency, thus he is now roaming Sanctuary Hills doing nothing instead of solving crimes. I believe the same is true of other followers - has anyone found a workaround for this? PC version.

Well killing Raiders with you is better that way.

Send him to the Castle otherwise and assume he's helping the Minutemen.