A bit late, because the battery indicators have been discovered (sometimes using a torch) a long time ago:
As a recent owner of the WH1080 I noticed that the Dutch / French manual offers much more information about the base station display than the English version(s):

26 Low battery indication base station
(lege batterij indicatie binnenunit)
28 Low battery indication transmitter
(lege batterij indicatie buitenunit)
Full manual:
http://www.alecto.nl/Images/Documents/W ... -FR2.0.pdf
Descriptions, translated NL -> EN with Google translate:
1: Time Reception indicator
2: indicator DST (DST) na. combined DCF receiver
3: DCF reception indication signal
4: Time Display
5: WWVB reception indicator signal (WWVB clock signal is American)
6: Alarm indication on / off (alarm)
7: display alarm time display
8: alarm time and date display
9: display date display
10: indication of the volume memory
11: Pressure history from 0 to -12 Hours or from 0 to -24 hours.
12: pressure indicator history graphic form ek
13: display current pressure
14: measurement of HPA (european) inHg (USA) mmHg (mercury column)
15: relative or absolute indication air pressure
16: indicate rainfall in mm or inches
17: display rainfall
18: rain amount per indication unit (1 hour - 24 hours - week - Months)
19: max indicate rainfall alarm
20: wind direction indicator
21: Wind Display Unit (m / s - km / h - knots - mph - bft)
22: windspeed
23: max wind speed alarm
24: display during contact with outdoor unit
25: humidity value
26: low battery indicator indoor unit
27: High / low alarm value humidity
28: Outdoor unit low battery indicator
29: Temperature display in
30: high / low alarms inside temperature
31: display display outside (temperature - wind chill temperature and dew)
32: outdoor display
33: High / low alarm outside
34: Control + key
35: trend indicator
36: On / Off button
37: Operation - key