Bob is, in fact, correct that his observation, "There's a new bookcase in my friend's house", is indeed evidence favoring the Bookcase Aliens\. Depending on how long it's been since Bob last visited that house, there might ceteris paribus be, say, a 1% chance that there would be a new bookcase there\. On the other hand, the Bookcase Aliens hypothesis might assign, say, 50% probability that the Bookcase Aliens would target this particular house among others\. If so, that's a likelihood ratio of 50:1 favoring the Bookcase Aliens hypothesis\.
"ceteris paribus" is an unusual Latin phrase in English. For clarity, a native English phrase may be better. Could go literal, changing "ceteris paribus be," to ", all other conditions remaining the same," or a bit more idiomaticly ", normally, be,".