"Key thob" and "key fob"

Solution 1:

As discussed above, this is clearly an error. However, it is an interesting error, which is driven by several different things:

1. The word fob is very rare, and is essentially unused outside of this expression and the idiomatic verb to fob off. Therefore it's not surprising that some people mis-acquire the word, since they have so few chances to correct themselves.
2. The sounds for th and f are acoustically very similar, and it is not difficult to mishear f as th. For a demonstration of this, see page 13 of this paper showing nearly identical frequency graphs for f and th [θ].
3. Because of #2, there are several English dialects where f is a frequent substitution for th, e.g. think is pronounced as fink. Speakers of these dialects have extra trouble knowing when to write f and when to write th, so many of the errors that you can find may be from people for whom fob and thob are actually pronounced identically. These people probably say fob, but mistakenly believe that the proper spelling is "thob".
4. Finally, the characteristic errors of the dialects in #3 prompt a hyper-correction from some people, causing them to "correct" f back to th in places where the f was actually correct. So some of the instances of thob may be due to people thinking that fob is a mistake, and that thob is the correct form.

Solution 2:

I get about 1000 times as many results on Google for "key fob" as "key thob". Google n-grams doesn't list a single occurrence of key thob.

I've never heard it used in any context, historical or otherwise; I suspect it's a mistake.

Solution 3:

Fob is correct. It comes from the term for a watch fob, a dangle or ornament that hung from your watch chain. Key fob, a device or ornament that hung from your car or door key, came to mean key chain. Then car manufacturers started using it for the auto-lock device that hangs from your keychain. The security key fob looks like the car start device, so people called it a key fob, especially as many people used to hang it from their keychains.

As for the f vs th sound, my husband used to pronounce the city of Philadelphia as Philadelthia, as he could not discern the difference in sound. So it happens in other words as well.