TL;DR: There's no visible symptom before you plug it in — the risk is that an unknown USB drive (found, gifted, or from an untrusted source) could contain malware that runs automatically or is disguised as a normal file. Avoid plugging in a USB drive of unknown origin at all if possible; if you must, use a computer that isn't connected to sensitive accounts, disable autorun/autoplay in your OS settings beforehand, and scan the drive with antivirus software before opening any files.
There's no visible symptom before you plug it in — the risk is that an unknown USB drive (found, gifted, or from an untrusted source) could contain malware that runs automatically or is disguised as a normal file.
What Usually Causes This
Malicious USB drives are commonly used to trick people into plugging them in out of curiosity, at which point malware can execute automatically or hide inside a file with a legitimate-looking name.
What to Do About It
Avoid plugging in a USB drive of unknown origin at all if possible; if you must, use a computer that isn't connected to sensitive accounts, disable autorun/autoplay in your OS settings beforehand, and scan the drive with antivirus software before opening any files.
If this looks similar to something else you've seen, it's worth reading Is My Android Phone Hacked? Signs to Check For.
This pattern shows up elsewhere too — see Is My iPhone Hacked? Signs to Check For.
Frequently Asked Questions
What are the real warning signs?
There's no visible symptom before you plug it in — the risk is that an unknown USB drive (found, gifted, or from an untrusted source) could contain malware that runs automatically or is disguised as a normal file.
What usually causes this?
Malicious USB drives are commonly used to trick people into plugging them in out of curiosity, at which point malware can execute automatically or hide inside a file with a legitimate-looking name.
What should I do about it?
Avoid plugging in a USB drive of unknown origin at all if possible; if you must, use a computer that isn't connected to sensitive accounts, disable autorun/autoplay in your OS settings beforehand, and scan the drive with antivirus software before opening any files.
Should I be worried if I only see one sign?
One sign alone (like a slower device) is usually not cause for alarm — most single symptoms have mundane explanations. Multiple signs together, especially unexpected data usage or unfamiliar apps, are worth investigating.