You're not technically in an encounter. Though they may search for you at this point, they may also just think it was an animal and give up.
The movement burrowing for a human is 0ft, because you must have burrowing speed to burrow.
Bssed on other research, powdery snow can be plowed through at about 1/4 hiking speed, walking was 50% saying burrowing or plowing is 25% is fair, so I'm going to let you push through your tunnel in this particular material at a rate of 120/4=30ft per 6 seconds.
Because you're not in an encounter, the limit doesn't apply.
In sand, no, you can't. In dirt no, in watery mud, yes but slower than swimming by half. In packed snow, no. In virtually any other material no unless you claim a burrowing speed and personally I don't like the mechanic. How can a monster "burrow" through dirt and rock at a number of feet per second?

I say no. Powdered snow, yes, at 1/4 movement if it's deep enough and loose enough.
Now, if said creature is in his habitat, expect that he/they/it/clownself could have a network of pre-dug tunnels and I will make a tunnel map in that case ahead of time. You may also do this, tunnels are a great way to get around especially when there is say, a dragon. But you will be limited to reasonable dig speeds, say feet per hour not per second.
Before I lead you too far astray, many digging monsters also have "tremorsense" so don't expect to get away with tunneling near a creature who has this.
>batista
Oh I see you didn't notice the batista, way worse than a ballista