If you’re pulling it out and putting it back in any appreciable distance, it’s probably already plugged. Also if it comes out without unscrewing it might just be a marker. What does the in ground end of the pipe look like? Is it threaded? It could be someone is just having a laugh…
If it’s only 1.5 feet then get a shovel and start digging. Get a gander for yourself what it’s connecting to. Sounds like it’s not connected to anything.