Maybe because on the phone you were using the FB app and you were logged in so you were able to find them somehow and when you where on the web browser you were not logged in to FB? Maybe log into FB on the web browser and search again to see if you can find them?
This is just a WAG.
I went to find a mutual friend on Facebook while on the web browser and couldn't find them even though I knew they had an account. Later that day while on the Facebook app I found this person. So I doubled check again the web browser and again not there. At one point I had my phone in my hand looking at the profile while looking back on web browser which still didn't show the profile. Why did that happen? Is there a new security setting for profiles?