I would ask will they be doing a PSMA PET scan first to identify exactly where the cancer cells are, so they ensure they're included in the salvage radiotherapy.
Otherwise they will probably just guess they're in the prostate bed (which is most likely), but if possible request a PSMA scan first to be sure. At 0.2, you are borderline if a PSMA scan will find them though.
Was the prostate histology suggestive of anything left behind? If so, that would add weight to it being in the prostate bed.