In the first two episodes of season two, Centurions crashed into Galactica. They then boarded the ship. I don't remember an explanation for how they were able to breach the hull (beyond initial impact, which was obviously noticed) to gain entry. Wouldn't their embarkation have caused decompression? (Sure, they don't need to breathe, but they need to open the hatchways to gain access from their crash-site, vetting the air.) Was this explained? Wouldn't the CIC or other areas have noticed an area decompress, then recompress? Was this addressed in the show? I don't have easy access to the episodes right now, but have seen them a few times and don't remember an explanation. I just remember everyone being freaked out that all of a sudden there were Cylons on board without warning.
They smashed into the Museum Pod, and their entrance was well noticed. One of the pilots even loudly confirms that there is no fire.
They smashed through the glass window sealing the runway, causing a decompression, and after a while they started leaving the Heavy Raider.
Also, this isn't exactly Star Trek. They don't have sensors for everything everywhere. They needed a pilot's confirmation to even confirm that the Raider hit Galactica and didn't miss it or smash into the hull plating. Damage control was very rudimentary and primitive, hull damage indicator was basicly several dozen LEDs linked to spots in important positions.
The rest is history.