House trained dogs having accidents

Question
My husband, my 3.5 year old mini schnuazer Gibb, and I have recently and temporarily moved into my sister's house with her husband and two small children (3 and 8 mos).  He is familiar with the house, he'd been to visit many times before we made the move. Even stayed for long periods at Christmas and summers. He's been doing great, good with the children and sister and husband.  We've been there about a month and last weekend while I was changing the baby and getting her dressed in the nursery he peed on the carpet right next to me!!  It was 1 pm and he'd just gone out around 10 am. Today my sister (she's a stay at home mom so she's home with him all day) told me that they were in the basement playing (getting his toys tossed for fetch) and he just squirted right there.  The door was even open to the back yard because the weather was nice and the 3 year old was running in and out!  When it was just us (me, my husband and him)he'd go all day while we were at work with no accidents.  We adopted him when he was 7 mos old and he was not totally housebroken but hasn't had an accident in YEARS!  We've even moved once before and he never had one accident the 6 mos. we were in the apartment.  (We are in the house shopping process... that's why all the temporary housing.)  I assume these are behavioral markings.  He doesn't have any other symptoms of urinary problems.  No loss of appetite, no straining, no blood, no lethargy.  He's active and loves his food.  He loves walks!!!  He does pee or lift his leg even when nothing comes out on walks.  He wasn't fixed when we adopted him at 7 mos so he'd developed the "marking" habit.  I don't know what to do, we were hoping to stay there until we found a house, but my sister will not tolerate ruining her carpet so we'll have to go if we can fix it.

Answer
This doesn't appear to be marking; it sounds more like anxiety driven behavior.  Although he's a marker (as you report), the sudden evacuation of urine as you describe is not marking. This dog is confused; even though he's visited and stayed for short periods (to a dog even two weeks isn't sufficient to become habituated), he seems anxious (his normal environment has disappeared.)  You need to reestablish house training by TAKING him out.  Do this religiously every three hours; do not expect him to use open doorways (this is NOT his normal environment, he may not be confident enough to go in/out in this manner...access to outdoors is more than merely making it available, to a dog the "right" to go in/out at will is a dominance related behavior.)  Praise him lavishly (and reward with tidbit) every single squirt he makes OUTSIDE.  Indoors put a belly band on him.  This is a humane device that straps around the penis; few dogs will attempt to urinate through it (the first time they try, the urine is contained against the body, something no dog will repeat voluntarily.)  If the belly band isn't sufficient, add a doggy diaper.

when you finally move, introduce this dog OUTDOORS and wait for him to eliminate before bringing him inside (no matter how long it takes him.)  Once indoors, reinstate the belly band or diaper for the first three weeks or so until he absolutely finds his "SPOT" outdoors.  Have patience.  For a dog to suddenly find itself dislocated (no longer in the environment where he knew his place, and now surrounded by other people) is very, very traumatic.  Even the most securely house trained dog WILL have accidents.