Sometimes a large shallow bubble appears from under the glass. If it has not thinned there are some things you can do.
First – do not drill holes.
One flattening method is to place the piece on 1mm to 3mm fibre paper and fire to a slump temperature. The fibre paper of these thicknesses will allow air out from under the glass. With sufficient time, the bubble will flatten. It will take some time as the weight of the bubble is slight.
Another method is to fire upside down. It does not matter whether the bubble is central or not. This will likely take less time than the first method, but requires an additional firing. To use this method, place the glass upside down on the shelf with an appropriate separator underneath. Take slowly to around 620C maximum for as long as it takes to flatten. A low slumping temperature will reduce any marking that later needs to be fire polished away.
When flat and cool, clean and fire polish.
If the bubble has become large and thin, this proposed process will not work. My suggestion for these is to avoid the effort to do an unsatisfactory repair. Instead use it for one of the many inventive process that use unsuccessful projects.