I was surprised to find a few signals were unknown via current_signal(Signal,Digit,_). Examining /usr/src/sys/sys/signal.h on FreeBSD, I found the following signal definitions that might be helpful in removing unknown types. The mapping between my swipl’s signal list and FreeBSD’s match on all, except unknown.
#define SIGEMT 7 /* EMT instruction */
#define SIGSYS 12 /* non-existent system call invoked */
#define SIGWINCH 28 /* window size changes */
#define SIGINFO 29 /* information request */