Unemployment rate can decrease because of fewer people seeking work, however, in Canada, right now, the labour force is actually growing. It hit some stagnation in '15/'16 but has been increasing since as Ontario's boomed and Alberta's recovered,
http://www.statcan.gc.ca/daily-quotidie ... 01-eng.htm
Looking at all labour force characteristics for London specifically,
From 2016 to 2017, unemployment dropped from 6.1% to 5.6%. Pop increased by 6,000, labour force (defined as those 15 or older who are employed or looking for work) decreased by 3,000, consisting of an 800 decrease in employed persons and 2,200 decrease in unemployed seeking work. So you are right that London did see a decrease in unemployment and a decrease in those looking for work. Is this people 'giving up,' or people moving away for work, with retirees moving in? There's all kinds of contexts that are hard to tell from this raw data.
In Ontario overall though and Canada overall, the workforce is growing, empirically.