After we flush the toilet water fills the bowl as normal, however after an hour or two there's no water left in the bowl. I've checked for leaks but have found none.

If it slowly leaks down and you see no water, the interior trapway probably has a crack and the water leaks down into the floor drain. It could also be a clogged vent and water running past the toilet pulls the trap dry. One way to check the internal crack is to fill the bowl, put food coloring in the bowl and then let it drain down. Pull the toilet, turn it upside down and look for color stains in the trapway.

There could also be a factory defect in the bowl. Sometimes in casting the bowl there is a small bubble in the china. Instead of junking the bowl the manufacture will put a porcelain patch over the hole and send it on through. On occasion the patch will fail allowing the water to seep out of the bowl. If that's the case you would have a full bowl after you flushed and the bowl filled but some time later you would notice a drop in the bowl level. The only other thing might be if you dropped something in the bowl and cracked it. If you're worried about where the water's going, in both cases it drains back down into the sewer and not under the bowl on the floor. To check the bowl must be pulled, filled and put up on saw horses. You can then see if there's any leakage from the bowl. Good luck, Tom

Good answers so far, this could also be a bad vent stack. If you stack is clogged out fixtures, when drained, can siphon the water out of your toilet. Does the toilet drain by itself if absolutely nothing else is being used?

If it were a bad vent you would hear a gurgling sound from the toilet bowl. Do you hear such a noise?
Besides it takes a hour to seep out. A blocked vent would pull the water out as another fixture drained or flushed.
My take on this is either a cracked bowl or a factory defect, Good luck, Tom

Agreed but we are still fishing here. I doubt if the OP stands over the toilet an actually watches it for an hour. It may be a hour before someone else flushes another toilet or uses another drain. Now if Tombel would come back with some answers maybe we could make some progress.