Bai Pu could guess this because he had speculated when entering the ruins that Garrison Academy wouldn't allow elite freshmen like Tom and Oshana to venture into these dangerous ancient ruins without protection.
It's important to know that the "Red Sand Kingdom Mausoleum" is just what the Desert People call it. At Garrison Academy, it has another name: it's called the "Naxitas Ancient Ruins!"
This is not some uncharted ruin; it's a mission set by Principal Bosh Gajisen.
Since Principal Bosh has made it an academy reward mission, it indicates that he has a relatively deep understanding of the ruins and has definitely researched the "Blood Moon."
Therefore, since Principal Bosh agreed to let the trio take on this reward mission, it's very possible that he would send experts to secretly escort and protect them.
