Message (single) Vanished from iPhone 4S

iPhone 4S

version: 9.3.5 (13G36)

Hello,

I've been trying to pin down an answer to this on my own, but I'm getting nowhere, fast.

A single text message has disappeared from my phone. No other texts have vanished, and none have done so for the many, many years that I've had this phone - I can still view (I think) every other single text I've ever sent or received.

I've read about texts vanishing from iPhones, but my circumstances seem a bit different from everything else that I've read. And now I'm afraid to plug it into anything or link it to my Apple account, just in case someone has remote access to it or I've accidentally allowed something malicious to be installed (maybe not as paranoid as it sounds).

That's essentially my question: Why has this happened?

Is this something common? If not, could some sort of "Wipe" program have been used by the person I texted?

That's the end of my question; but here's some context, on the off chance that it's helpful:

I have an old iPhone 4GS that I occasionally throw some pay-as-you-go minutes on (I don't often need a phone).

I recently added some minutes, and texted someone I hadn't talked to for about 3 years. This person's names and number was still in my contacts, along with an old string of text messages. Because I assumed the number still belonged to this person, I navigated to the old text chain and sent a message. I received a reply in the form of a jpg-meme thing - which I opened and looked at - and it seemed in character with this old friend of mine, so I assumed such was the case. I fell asleep before replying.

The next day I had a missed call from a number I've never seen. I texted ? to it. I got a text reply that was just a grey bubble (which I very wisely clicked on, by the way).

The following day I had another missed call from yet another number I've never seen. I texted ?. A sincerely perplexing conversation ensued. I won't post what it was, but I engaged because I assumed it was my old friend, texting from a different number.

Within the next 2 days, the text with the blank bubble had vanished, and a new unfamiliar number (a fourth, if you're counting) showed up in my history. I've never called this number, and I'm nearly certain it has never called me, but its appearance coincided with the disappearance of the grey-bubble text. Anyway, WTH?

I've also had odd battery power flucuations, and a seemingly higher battery drain rate, lately, but perhaps that's not so unusual.

Thanks for any help or insight.