Simon did not have any mechanical aptitude as such, but he could follow directions easily enough and there was a lot of work to be done to rebuild Serenity. Simon could do whatever task Kaylee set him too though most of these were in her domain of the engine room where she could keep an eye on whatever she set him too. It was also mostly private so when they got distracted from their work - he was sure Kaylee had got engine grease on his shirt deliberately but he'd only laughed and discarded it and been teased in a way that made it clear Kaylee approved of his decision.
But she'd given him an engine part and told him to strip some wires which was simple enough and he'd just picked up the tool and turned around to get started when he was no longer in the engine room. He was on a dusty road, on what seemed to be a rim world. He blinked and rubbed his eyes and then looked around. He started to think about what medical conditions could cause blackouts or memory gaps - but he was still holding one of Kaylee's tools. And if the ship was repaired enough that they'd gone somewhere - or even if he'd disembarked, he can't imagine any circumstance in which he wouldn't have stopped to retrieve his shirt.