A couple years ago I had an OFS who was super productive.  
She’s really smart, perfect English, willing to come to me with suggestions. Just really great.

Then over a couple month period I noticed her productivity dropped. 

I didn’t say anything for a while, but I wondered “did she get another job?”

One day I got an email from her which was intended for her other employer.

She was really embarrassed.

I was grateful to finally know.

Her and I talked through it.

Why she got the other job.
Her financial situation.
Her work with me.
What we’re going to do moving forward.

She still works for me.  She makes more now. Her productivity is back up.

I’ve found out my OFS have other jobs 3 ways over the years:

1. I get an accidental email intended for their other boss
2. TimeProof. Screenshots show work for someone else 
(I don’t use TimeProof but did years ago…I just don’t like it)
3. I asked and they told me.

To me someone getting another job is an indication of one (or a combination) of the following:
– I’m not paying enough 
– I’m not keeping them busy enough
– I’m not a good enough boss and they’re worried about losing their job
– They’re more ambitious
– They’re bored with my job (I’ve created a dead end)

I’ve always been able to work it out with them and keep them working for me.
Sometimes their other job fizzles.
Sometimes I incentivize them to just work for me.
Sometimes they maintain both and keep their productivity up (this usually doesn’t last super long).

Again…asking them is key to me.