I've not run into this. I know of no way to depict it using ISA symbols unless you use X, which is the unclassified alpha character. X is intended to cover unlisted designations and seldom used. If it were me, I would put a note by the HH alarm and cover that in the notes on the P&ID and detail it on the loop sheet, if needed.
I'm sure others will have much better ideas.
Pamela K. Quillin, P.E.
Quillin Engineering, LLC