Immobilizer test 2003 986 Std/Tip

[webwalker]

Hi all. I've been driving my MY03 since Sept 2020. Perfect.
Replaced the battery June-2021. (Was 7yo.) No problem.
Vehicle is climate control garaged. The top has never even been wet.

Parked in my shop for 6 weeks with battery tender, when the weather got weird
12.8v when taken off the tender this past weekend.
Key to RUN, fuel pump primes, all dash lights on, just as always.
Key to START....and nothing changes. Not even a 'click' at the solenoid. Dash lights don't dim.

I've run the resync procedure for the keys. Lock, unlock, top, windows, frunk, trunk, everything works as if it rolled off out of the factory. Amps look good from the battery.

Next steps are testing the starter relay at position 7 at the trunk relay bank.
Then check continuity on fuses.
That's when I run out of ideas. I'll spend to fix the vehicle, but I don't spend when there are additional diagnostics I can do without the spend. I'm looking for next suggestions, as well as a test I can do to confirm the immobilizer is working without dropping a financial bomb on testing equipment.

I've got 30 years working on cars, but most of them have been vintage, or didn't have immobilizer systems that were this....ahem, strident.

Suggestions?