What is this pex tubing?

[Mo3]

What is this pex tubing under the bathroom sink? That is an external wall. The other pipe going out the same way is an oudoor faucet. There is a bad small from that tubing. The sink water has been turned off for several years. It is an unused bathroom

 

[PlumbGate]

Bad smell means it is probably a vent homeowner added). I would plug it and see if any other drains in the house are affected.

 

[Geofd]

try filling the trap with water and see if the sewer???? smell goes away