Jan - ATC automatically provide an Alerting Service if you are in receipt of a FIS regardless of whether or not you are on an FP, so you would presumably of got that call in any event.
As to Q regarding the point of an FP if SAR is not automatically launched, consider some of the mandatory requirements for FP filling:
Crossing International Boundary
Entering or leaving controlled airspace.
Neither of the above 2 categories are specified just for locating overdue aircraft, as it is of course just as serious if a local OCAS flight goes missing and no FP is required for that.
Whereas, for direct safety issues encompassed by for example:
More than 10 miles from a coast
Crossing dangerous and/or remote terrain
FP's are only recommended and are not mandatory.
This suggests to me that FP requirements are more geared to admin than safety requirements.