A 1:1 is likely a false positive. Did they do a confirmation test? If they did, and the confirmation test was positive, then it isn't.
Of course, that would be really confusing because your tests after the penetrative encounter were negative, though the rapid test isn't as accurate as a blood test. Still, a 1:1 is really low, and I'd expect it to be higher.
Syphilis is transmitted by direct skin to skin contact - unclothed oral to genital, genital to genital, or anal to genital. It is not spread by hands.
If you can find out exactly what tests were done, that would be really helpful. It would say something like VDRL or RPR, then FTA-ABS or TPPA or TPHA.