A junior level technician observes a senior technician using a pen-like device. What might the senior technician be doing?

The senior technician is likely using a non-contact voltage detector. This tool allows technicians to safely identify the presence of electrical voltage without making direct contact with the conductors. The pen-like design is typical for non-contact voltage detectors, making them easy to handle and maneuver while performing checks on electrical circuits or equipment.

When using this device, the technician can quickly assess whether wires are live or not, which is crucial when troubleshooting issues or performing maintenance in a safe manner. The visual indicators (often in the form of LED lights) provide immediate feedback, enhancing the efficiency of the technician’s work.

The other options involve functions that do not accurately represent the capability of a non-contact voltage detector. For example, while recording modulated signals or making invisible cable markers are tasks that may involve specialized tools or techniques, they wouldn't typically utilize a pen-like device in the same straightforward manner as voltage detection. Charging a USB battery does not relate to the specific function of identifying electrical presence either. Hence, the use of a non-contact voltage detector aligns perfectly with the observed scenario.