I always close the main valve during vacations of more than a weekend. Every time after returning, when I reopen the valve slowly, the water flows for quite some time to what I assume fill the empty pipes. The meter needle is in same position as when I left so the valve is closed for sure.
So where does the water in piping go? There is no visible leak anywhere and no water was used after closing.

Before turning the valve on next time, look at the water level in the toilet tank(s) and see if the tank(s) are full or leaked down.

A common place for it to go is in the toilets tank, a slightly leaking flapper will allow the water level in the tank to drop enough so that the fill valve will open and try to replenish the water.

It's filling the toilets...the water evaporated.

Thanks all. The toilets do seem the likely slow leakers over the 5 days we were out. Though nothing is visible.
Is this normal or should the toilet flaps be replaced? It must be an awful lot of wasted water.

I'd replace them if you think they are leaking. They are low cost, easy to change. Plus you are paying twice for that wasted water, once for the water and again on your sewer charge that is on the bill.

put some dye into the tank and see if it seeps into the bowl.