Most buyers of a residence will have an inspection done. While the inspection report looks to be thorough, and while they are better than nothing, generally they are not very good. One thing they rarely if ever cover is rats and scorpions, and even the obligatory termite inspection will not cover rats and scorpions. You should strongly consider having a firm that specializes in roof rats inspect the place. Here is also what you might consider doing when you are on the buy-side of the transaction: (i) Ask the seller if the house has been sealed for rats and scorpions; (ii) if the answer is "yes," end of story, but if the answer is "no," ask for a price reduction to cover the cost of a rat/scorpion sealing; (iii) if the seller agrees, you're good to go, but if the seller refuses, tell the seller you want the home warranty to include a guarantee against the presence of any rats or scorpions for the term of the home warranty with an appropriate remedial clause should rats or scorpions show up. As in any negotiation, you might not get all or any of what you want, but as in any negotiation, it doesn't hurt to ask.
PS: If you having your house sealed against rats, if you have any reason to believe after doing the trapping and smoke test that any rats might still be in your house, be sure to alert your sealing company. They will then do a partial seal, sealing up all but one or two remaining spaces which are then heavily trapped, to be sure no rats are sealed in before they finalize the sealing.