Apparently, after I did some googling, the Samsung S5 uses a
proximity detector from AMS called a TMG399x (I'm not sure exactly which x).
The TMG399x is a pretty interesting unit that can be configured using I2C to emit varying width pulses of IR at about 860 Nm (well, that's the peak "normalized responsivity" of its IR receiver anyway)...and it can detect "gestures" using the built in North/South and East/West IR detectors.
Given that the TSOP4838 expects IR bursts at 38 kHz I thought I wasn't seeing the whole picture, so I hooked up an IR photodiode in parallel with a 4.7 k? resistor and checked it on my scope. The TMG399x was emitting 2 x 620 ?s bursts of 12 pulses at about 19 kHz with a 32% duty cycle every 5.8 ms - CONSTANTLY.
This is kind of what the TSOP4838 was reporting (except it treats a burst as a low and silence as a high), so I guess the TSOP4838 is fairly tolerant of the PWM burst frequency.
So far I haven't found a way of turning this device off on my phone (if only to see whether I'd miss it).
Edit: it would be good if the forum could display unicode characters properly