Propane Tank Issues

Photo please
I will add the Fill port is kind of large... it is outside thread (The fill nozzle fits OVER it) I'm not sure why you'd call it female save the hole in the middle... There is a check valve behind it.

There is a three 'Fingered" knob valve beside it (The Visual Overfill Indicator Valve" if the fill tech did not fully tighten that will leak you will Smell Propane if that happens. (You may for a bit even if the valve is properly closed as some propane is vented during filling)

THe other fittings on the tank are smaller

IF you have a gauge on the tank the Fill port is, as I recall Larger. (The other ports smaller).

If it's at the bottom, it might be a liquid withdrawal port, sometimes used for LP generators.

That female threaded fitting is the hI PRESSURE fill nozzle. The Threads look new(clean) because this fitting usually has a yellow cap installed. You remove the cap to install the fill nozzle. Since NO yellow cap, then it has been lost. Obvious cause of no LP after fill is the filler did NOT fill the tank full. If it leaked out the usual 15 to 25 gallons of LP in a few days you would hear it and smell it. You need to get a cap to cover the fill nozzle. This link shows what it is. Doug

https://www.amazon.com/Marshall-Excelsior-ME109-Tank-Valve/dp/B007HRKQ2Q/ref=asc_df_B007HRKQ2Q/?tag=hyprod-20&linkCode=df0&hvadid=312191829731&hvpos=&hvnetw=g&hvrand=7850837316716751630&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9027230&hvtargid=pla-571603853261&th=1

could it have been a stub for a propane bbq and somebody stole?

Some propane setups come with a manifold where the LP can split off to furnace, hot water heater, stove, oven and external bbq. Should there be more outlets than needed the manifold may have a "plug" on an unneeded outlet. If that plug is loose or missing then you would lose LP from it. Maybe post a pic for folks to look at