Let's compare that to RKBA
Let's.
Presence in an area of prohibited activity (shooting people without justification) + admission to doing something similar (carrying for self-defense) + tool (firearm) + evidence of recent use of tool (it's being carried on person, dirty grips, whatever - although I don't think "evidence of recent use of tool" even applies in the OP's circumstance).
(I believe the OP said, or at least implied, the trowel was dirty from some gardening.)
And, for the record, analogies are always imperfect.
I think the RKBA formulation would be more like this:
Presence in area where illegal shootings happen regularly (the classic "high crime area") + admission to doing something similar (like target shooting in the area) + evidence of recent use of the tool = at least RAS for stop and minimal search of the car, as the container for the tool.
There is zero evidence of any illegal activity here.
I think a court would disagree. Agree to disagree.