Yup... No Tool, No Plug, just an empty hole.
If you think it needs a plug of some sort, you'll need to make one yourself out of silicone glue (or something similar), as nobody makes a plug for the Shield (yet).
[Edit] or buy some NDZ Performance mag base plates. They cover the hole.
Thanks to all. Yes, the 8 round mag covers the hole, while the smaller one doesn't. Kind of weird; I never looked there, just slapped in a fresh magazine.