Nancy Guthrie: Pool Cleaners Show Up at Her House Amid Police Probe

12:39 PM PT -- Pima County Sheriff Chris Nanos tells TMZ ... the Guthrie family requested the cleaners come by to maintain the pool.

Another strange development in the search for Nancy Guthrie ... pool cleaners are hard at work at her Tucson home after her alleged kidnapping.

Several reporters on the ground in Arizona shared video of work trucks from the local company Ambiance Pool Service & Supplies in front of her house. Drone footage posted by MS Now reporter Alex Tabet shows two guys dragging nets through the water in her backyard.

It's unclear why they're doing this ... but law enforcement still has the house locked down -- the tweet above shows at least one deputy escorting the pool cleaners to the front door.

As you know ... Nancy was last seen nearly two weeks ago -- returning home after having dinner with family on Saturday, January 31. She was reported missing the next day after she missed church.

We received an alleged ransom letter just days after she disappeared, which mentioned two deadlines ... both have since passed.

The FBI has released multiple videos from the area on the night Nancy went missing ... including shocking doorbell footage from her home, which captured a man in a ski mask and backpack at her door. Federal authorities say the male suspect is of average height and build, and he has facial hair.

We've also learned cops are taking a hard look at footage from a camera taken about a week before Nancy's disappearance, which shows a strange man approaching a door with his back turned about 6.5 miles from Nancy's home.