I wouldn't expect $ nrpe -f to write a PID file, since it's started by a normal user (not root) and in "foreground mode", but apparently it still does. Or at least it tries to and bails out upon failure.
Please consider fixing the behavior of the -f switch.
According to the sample config file
https://github.com/NagiosEnterprises/nrpe/blob/56492da4e4b55c8d188ed8114a88c18f0250ff55/sample-config/nrpe.cfg.in#L39-L41
I wouldn't expect
$ nrpe -fto write a PID file, since it's started by a normal user (not root) and in "foreground mode", but apparently it still does. Or at least it tries to and bails out upon failure. Please consider fixing the behavior of the-fswitch.