I've told this story on r/ActLikeYouBelong long before and just now realized y'all over here might enjoy it, too - so here is the tale of how I accidentially infiltrated a high security site:

This story is from my early years in the workforce. I was around 20, working in IT as a system administrator, when the company I worked for still had a tiny branch office on the huge factory grounds of a very successfull industrial company. This branch office only existed for reasons of a historical partnership between said company and another one we merged with years ago.

On the day this story takes place, this branch office had been officially closed for good. I was tasked with dismantling the network infrastructure installed there, holding a quick debriefing talk with the other company's sysadmin team, taking inventory of all our devices and bring them back to HQ. A facilities coworker who was sent with me had the task of collecting the remaining furniture.

Said coworker and me planned our trip, rented a van and made the 2 hour trip there. For context, the rental van we got was a white Fiat Ducato, Peugeot Boxer or Citroen Jumper - visually almost identical, all built by the same joint venture, millions of them are on the road and used by companies of all trades. They are the default work van you can find in Europe. My coworker and I were both dressed in high quality worker's cargo pants with some tools in it and a hoodie - pretty much the unofficial uniform of the European tradesman. In short, we accidentially had the perfect infiltration disguise.

Here's where the unexpected fun begins... at the gate of the factory grounds where our tiny branch office was on. Security was very tight there, as this industrial company manufactured goods there that were world class and they had caught spionage attempts before. I knew the security measures from my former on-site job visits and had prepared all required paperwork and IDs earlier, neatly packed in the glovebox.

Unbeknownst to us, at the last traffic intersection we had ran into a convoy of three similar work vans, who also all happened to be headed for the industrial company's grounds... So we found ourselves stuck between them 2nd in row in a neat line at the front gate. The security guard checked the van in front of us before opening the gate. When I attempted to stop in order to show security my paperwork, the van behind us - belonging to the same electrical company as the first and last one - violently began to honk. The guard glanced to him, back to us, and just impatiently waved us all on! Our van was the same type, just brand new and therefore probably looked to him like it just hadn't received its corporate branding yet. Alright then, so I pulled through the gate and played part of the convoy. Soon we reached an intersection on the huge factory grounds where I had to take a left turn to head to the side building where our tiny office was located. When I switched on my indicators accordingly, the security guard by this intersection firmly shook his head "no!", demonstratively closed the road I wanted to take, and gestured to follow the other vans. My coworker and I began to feel uncomfortable about this weird situation.

Our convoy came to a stop by a huge manifacturing building. The electricians in the other vans quickly got out, looked confused at us, we looked confused at them. The sparkies shrugged and assumed we were here to perform some other work, fetched their tools and got ready to leave together. My coworker and I had only fetched our paperwork from the glove box, intending to look for some official or guard we could talk to fix the misunderstanding, but nobody was outside the building... So we followed the electricians.

It was several minutes after we were let inside the manufacturing hall that we found a security guard to talk to - and we expliclty had to get his attention. We could've just kept on walking behind the electricians and infiltrated the top secret area beyond all we liked. The guard was surprised, radioed his supervisor and guided us outside back to our van. His supervisor met us there, questioning how we got here and what business we had on the premises in general. Producing our paperwork and explaining the mixup immediately defused the situation. He escorted us to the building of our tiny office using his car, and that was it. We got the job done as planned and left... Still wondering however if gaining access to a high security industrial site could really be as dead simple as looking like tradesmen and piggybacking to a convoy of other work vans heading inside.