A tiny and previously unknown portrait of Auguste Rodin has been discovered thanks to an unwell child, a sharp-eyed art graduate and an unusually constructive use of Twitter.

Luis Pastor, a Spanish graphic designer who lives in Luxembourg, was visiting his parents in Madrid last week when his eldest son fell ill.

After his mother volunteered to take over, Pastor made for the nearby Lázaro Galdiano museum.

It was there, in a dark room that holds the museum’s miniatures, that he came across a face he had known since his days as a fine arts student.

Before him, in bushy-bearded profile, was Rodin. The only problem was the caption, which described the painting not as a portrait of the French sculptor, but as that of King Leopold II of Belgium, a monarch notorious for his brutal administration of the Congo.

King Leopold II of Belgium. Photograph: W and D Downey/Getty Images

“I got to the last picture and thought I’d misread the caption, because I recognised who it was straight away,” Pastor told the Guardian.

“I love Rodin and have been to the Rodin museum in Paris a lot. I was obsessed with him as a student. I started Googling pictures of Leopold and thought ‘They do look like each other but that’s not Leopold.’”

After leaving the museum last Friday, the 39-year-old designer continued his research. The deeper he delved, the more convinced he became.

“It was the colour of the eyes that sealed it: Rodin had blue eyes and Leopold had dark eyes. But it was also to do with the shape of their ears. I spoke to a couple of friends who are historians and restorers and they said: ‘Come on. Why are they going to have a miniature of Rodin?’”

But, as Pastor points out, José Lázaro Galdiano – after whom the museum is named – was a compulsive collector who snapped up anything that caught his eye.

Pastor tweeted his findings as he went along, sharing them with the museum.

“The museum said they’d talk to their head curator but I never expected them to get back to me so soon,” he said.

By lunchtime on Tuesday, Pastor had been proved right.

“We now have a definitive verdict,” the Lázaro Galdiano tweeted. “After numerous comparative analyses of portraits of both men, the miniature, catalogue number 3711, turns out to be a portrait of the sculptor Auguste Rodin. Many thanks to Luis Pastor for bringing it up!”

The museum said it welcomed any further suggestions about its collection, adding that pre-internet attributions might need to be changed.

The Rodin museum tweeted Pastor its thanks and congratulations on Wednesday morning.

“I feel a bit like Sherlock Holmes,” he said.

“That painting’s probably been there for 50 years and no one had figured it out. It’s a bit silly because it’s a tiny picture in a tiny display case in a really small museum. But it’s a picture of one of the most important sculptors of the 19th and 20th centuries.”

Pastor said his discovery of the treasure was doubly sweet – as much for who it was not as who it was.

“I also feel a tinge of personal pride at showing it’s not Leopold, who was responsible for a genocide. That’s not bad is it? A win-win situation.”

A rather more famous artistic mystery appeared to have been solved last year when experts claimed to have found the model for Gustave Courbet’s L’Origine du monde (The Origin of the World).

The painting, a close-up of the naked thighs, genitals and torso of a reclining woman, was thought to have depicted Courbet’s lover, the Irish model Joanna Hiffernan.

But new evidence suggests the reclining figure was in fact the Parisian ballet dancer Constance Queniaux.

The dancer was a mistress of the Ottoman diplomat Halil Şerif Pasha – aka Khalil Bey – who commissioned the painting from Courbet for his personal collection.

Nine months ago, a British art historian claimed to have established the identity of the man who sat for Vincent Van Gogh’s Portrait of a Gardener.

According to Martin Bailey, the subject was Jean Barral, a gardener who worked in the area around the Provençal asylum to which Van Gogh was sent after cutting off his ear.