Ultrasound - lump felt but nothing on scan

My 2 year old son has a small lump just below the back of his knee. It’s less lump lump, more squishy area. Took him to the GP who felt it, said she wasn’t sure what it was but didn’t feel it was anything to worry about, but referred us to paediatrics to be safe.

Had an appointment with a pediatric orthopaedic surgeon two weeks later. They felt it, observed him walking and asked me some questions about it and said they were sure it was benign, and most likely a cyst. They referred us for a Non-Urgent ultrasound to confirm which he had last month, 7 months after we initially went to the GP.

The ultrasound showed absolutely nothing. No abnormal areas at all. This threw me. The orthopaedic surgeon had another surgeon review and agree that they had no reason for any follow up, it could be a lipoma, and effectively left me with ‘if it changes come back to us, but otherwise there’s nothing to see here’.

Now, I’m trying to be comforted by the fact that sinister things generally show up as something on an US, but my anxiety is spiralling. What if it’s the super rare Synovial Sarcoma which is slow growing and seems benign at initial exam? But surely that would show up? Or what if the sonographer made a mistake? He was upset during the scan but she did see the area and his leg is only small!

I’m just, terrified it’s something more sinister due to lack of a concrete answer. Does anyone have any experiences similar to this, where they had to just live with a bit of the unknown and trust the medical professionals?