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We moved into a newly built house about half a year ago. A few months back we started noticing a (rodent) urine smell coming from a specific point at the floow near an outer wall. We also started hearing scratching sounds coming from the same direction. We haven't seen any mice or droppings anywhere inside the house, so I'm fairly certain they are only inside the cavity wall. Since we haven't leveled our garden yet, some of the foundation blocks are still visible from the outside. I assumed this is where they were getting in the wall.

I checked the outside of the wall where the sounds and smell were coming from and placed some poison in some of the openings. None of the bags were ever gnawed.
When that didn't help, I sprayed expansion foam in the openings instead, covered all the visible blocks with sand and tamped it all down so I could see it if it got dug open again.
This seemed to have helped at first, but since a few weeks we can smell and hear the same things coming from another wall nearby, this time an internal one (that is connected to the outer wall where the issue began).
This time I bought one of those ultrasound emitters which are supposed to drive them away. I also covered all the nearby foundation blocks with tamped down dirt so I could find out which way they would flee. I was hoping this would help me to find their entrance so I could properly close it off.

However, the emitter has been switched on for about 2 weeks and there is no difference. I can still hear scratching coming from inside the wall and can't find any digging outside the walls. I would assume that if the mice are locked inside, they would eventually starve, but this doesn't seem to be the case since we can still hear scratching.

Is there something else I can do to find out how they are getting inside the walls? It's not like they're surviving on eating the insulation right? (Or eachother?)

TLDR: Can't find entrance of mice in cavity wall, closed off all nearby holes, installed ultrasound emitter.

Teebs
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    Most of those so called emitters are snake oil. I think only a few of hundreds have been proven to work. They say mice only need a quarter inch hole to get in. Seal all holes with metal(heavy screening or thin sheets) or cement/concrete/mortar. – crip659 Oct 23 '23 at 12:51
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    Rodents will eat spray foam. They'll eat insulation (foam, fiberglass, PVC on your wiring, anything). They'll eat drywall, wood, plaster. They'll survive inside your house just fine, thank you very much. Since this is a newly built house, can you call back the contractor who built it to have them fix up the problems? If there are holes in an external block wall, they need to be properly patched with mortar (unless they are weep holes designed to allow moisture out). – FreeMan Oct 23 '23 at 13:00
  • If they starve in that cavity, you're going to yearn for the days of mere urine smell. Poison is a bad strategy also for this reason. A 3/4" hole is plenty big for a mouse escape and easy to patch. I'd drill a 3/4" hole in the wall and tape a plastic tub over the hole with a baited mousetrap inside. You want those rotting little wormy corpses in the landfill, not your wall. – popham Oct 23 '23 at 19:20

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