Perhaps it is a young plant of one of the dwarf lilacs that still grow at least 5':

If it blooms before common or French lilacs -- Syringa pubescens ssp. meyeri Palibin

If it blooms with or just after -- Miss Kim (Syringa pubescens ssp. patula). The leaves are different -- shiny, slightly elongated leaves.

A quick look at Monrovia's website might help you find the one you've seen.

Here is a link that might be useful: Monrovia lilacs

'Miss Kim' has been real popular the last few years.
I have one in a 5 gallon pot.
I'm not too fond of Lilacs on this side of the mountains. Not enough bang for the buck. They prefer sweet soil, hot summers, and cold winters. They don't bloom here like they do in Spokane. Fall color is zilch, and the branching pattern leaves a lot to be desired. (Think vertical brush pile) The fragrance doesn't seem to be as intense as well.
I almost forgot, there's a leaf roller that makes them look bad all summer.
Come and get it.
Mike....getting finicky in my old age.