Ok, so poking about more with SWFOTEFUSS: I've discovered this:
If someone is checked for force by a player/imm/whatever using the fskill sense victim->force_identified is set to -1.
The setforcer command only sets victim->force_chance to be 1, and doesn't reset the force_identified back to 0, if its not 0, then sense will fail and you'll have to edit their pfile manually.
Find code in force.c
if( !str_cmp( arg_flevel, "force_chance" ) )
victim->force_chance = 1;
if(victim->force_identified == -1 && victim->force_identified != 1)
victim->force_identified = 0;
If I recall correctly, 1 is you were sensed but don't know what it is.
What we are doing here is resetting the value of force_identified back to 0 if it was set to -1 by the skill sense or any other means.
I think that should do it, however I didn't spend much time looking at this so might be a good idea to double check.