You have a very small risk of pregnancy, so small, that I would not really worry about it. Basically, pregnancy would only be possible if you ejaculated very close to her vagina and some semen dripped into it, or if you rubbed the tip of the penis on her vagina when pre-ejaculate was coming out. If you both had underwear on, that makes the chances even smaller as sperm has a tough time swimming through fabric. The fabric would pretty much have to be soaking wet for the sperm to get from one side to the other since sperm has to stay moist in order to live.

By contraceptive 'pads' do you mean the patch? If so, then she is still protected in her 'patch free' week

pre-ejaculatory fluid normally doesn't contain any semen. Because urine and semen go through some of the same passageways and it is a slightly acidic substance, pre-ejaculatory fluid is expelled to neutralize the pathway for semen.
Also, chances of pregnancy are very very slim to be fretting over if you still had your boxers on and it actually was semen. however, you may think about using a condom anyway. It's always better to be safe then sorry when the temptation is there.