A woman was left appalled after finding a message written to her by a mechanic after sending her car to the garage.

Hidden notes are often a nice sentiment from loved ones, found in your lunch box when you were in school or from your partner on your first day at a new job.

But this incident proves that not all hidden notes are kind, as this woman was on the receiving end of a brutal put-down after she sent her car for a simple filter change.

The woman's partner posted the incident on social media and it quickly went viral as other mechanics weighed in on the incident.

They put the post up on Reddit, captioning the photo: “My fiance's air filter installed during her service last year…”

Upon closer inspection, the filter had a message written on it, reading: "You are FAT".

The couple didn't reveal if they found the message funny or insulting, but users on the social media site were quick to share their own takes on it - with many being mechanics themselves.

One person wrote: “Low volt electrician checking in. Can confirm this is the sort of thing you'll find virtually anywhere semi-hidden from the public.

“Inside A/C units, duct work above a soft lid ceiling, scribed on the steel pillars of new buildings, on the side of concrete Christy boxes. I, myself, like to draw dumb pictures like stickmen on islands saying ‘Omg you found me! Thank God! I'm saved!’.”

A different user shared that their friend did the same thing as a mechanic, but stuck to middle fingers or 'wangs' - just the classic stuff then.

While a third commented that one of his mechanic friends discovered a message through this method, advising him not to serve the customer: “My buddy who’s a mechanic has a story about finding a note from a previous tech hidden behind the air filter that said 'just put everything back, this guy won’t pay for anything.'"

As was predicted, when the owner was told what was wrong with the car, he refused to get any work done, before leaving.

Another user posted: "I have 10 year old car that has had some work done on it over the years. Now I wonder if I'm driving around with secret messages."

But it's probably nothing malicious, as one user pointed out.

"If you aren’t a mechanic, never been a mechanic, or related to a mechanic you would think this is for the customer," he began.

"I assure you this is not, the mechanic likely never saw who the customer was and if they were to do some passive-aggressive s*** it would be towards the dumb*** service advisors, not the customer. This was clearly intended for the next mechanic to find," they added.

Don't worry if you get insulted at your next service then, it's nothing to do with you, according to the writers themselves.

LADbible has contacted the mechanic service for comment.

Featured Image Credit: Reddit/Getty Stock Images

Topics: Social Media, Viral, Reddit, Cars