Thank you for your assistance and analysis Darryl.

I ran the valicam2 utility against the igc file and it checks OK:
Log Data Integrity OK
Signature Data Integrity OK
Security OK

The internal battery has never been replaced, as the unit is just over
two years old (Last calibrated 9/5/08 at the factory)
The C302 showed 'Seal OK' when I turned it on, pretty sure of that,
but will check again next time.

I'll check the antenna connection before powering on next time, and
hope that you are correct and this was a on-off error.

Regards, Bernie