has anyone else ever found their call forwarding was activated and call forwarded to a strange number

Only when I lent my phone to so-called "friends".....

If your iPhone has not been jailbroken and has the latest iOS version, it is extremely resistant to such remote tampering, except if the Federal Government considers you a Person of Interest. If the phone was jailbroken, all bets are off, as is the case of Android junk.

Okay, it was worth a shot. It's also possible (yes, I know I'm getting far fetched now) to change your call forwarding number by using dialing commands. While I doubt that a child can randomly invoke a sequence of numbers that happen to work out to a local number, it is possible that a pranking friend could borrow your phone 'to make a call' and do it literally in seconds.

Depending on carrier, you just can dial (from the phone app), *004*1234567890# then hit send/dial on Rogers Wireless (my carrier) and all busy and no answer calls will go to (123) 456-7890.

Call forwarding numbers are I think stored on the SIM (not 100% sure) but they are not actually stored on the phone, it's a carrier function. You can tell, because when you go into the phone app to change your call forwarding numbers, there is a pause/hesitation because the phone is contacting your carrier to retrieve those numbers so that it can be displayed on your phone. The litmus test for this (on my previous devices, not sure about iphones) is to go to a no-cellular coverage area and try to read your call forwarding numbers on your phone, basically you can't.

BUT all that said, I haven't used the iphone's settings app to change the forwarding number -- I did it just now just to test it out, and I do see the hourglass momentarily -- but I think this setting is for Unconditional Call Forwarding (forward ALL Calls to the number specified) where it does not even ring on your device (as opposed to the busy / no answer Conditional Call Forwarding).

It's possible also that forwarding is to a now-defunct voicemail number. By default for most phones, unanswered calls, unavailable for calls, and busy get forwarded to your voicemail. If you have "conditional call forwarding" enabled through your carrier, you CAN actually have each of those three conditions forward to three different numbers. Enabling conditional call forwarding is a must when using a 3rd party voicemail service which is not your carrier's voicemail (e.g. YouMail Visual Voicemail). For YouMail, you are given a custom local phone number to forward your unanswered/busy/not-available calls to. If later, you unsubscribe from YouMail, you must remember to change your voicemail conditional forwarding back to your carrier's voicemail number, otherwise when YouMail deactivates that number, your callers will get a not in service message.

Probably unlikely in your case but YouMail for iOS is available in the app store, so the whole scenario is possible, although maybe not likely. Does/did your voicemail work? Did the SIM come from a previous phone (I think that voicemail numbers are stored on the SIM)? Have you ever tried a 3rd party voicemail service?

To clarify; call forwarding sends all three types to one phone number. Conditional call forwarding can send all three types to one phone number or each type to it's own (different) phone number.