Jawfish are hole dwellers. You usually only see them sticking their heads out of their holes and at feeding time. Invariably, in tanks with a lot of rockwork, they dig their holes on the backsides of the rocks where you can't see them.
What I have done in the past in tanks without a lot of substrate was to use a large deep tupperware container placed near the front of the tank, in front of some rocks to offer protection, then fill it with soft sand and make a depression in the sand and you will find the jawfish will make their hole near the depression. ( Some have used a piece of PVC
pipe as the hole and introduce the jawfish into the tank right at the hole.)
For now, just keep your eyes peeled for depressions in your substrate near your substrate. The jaw is probably somewhere nearby.
Hope this helps.