As far as I know never have seen in relationship to liver flushing. Read it on other forums. It is often from not being able to digest proteins well, but it can be a lot of other things too. Safest thing to do is get a doctor's urine test at least you will know what you are dealing with.
May be it is from certain supplements. Did you do a search on All forums, because it has been discussed in different forums. In the beginning of iodine, may be 5+ years ago, it came up too but I can't remember if it was related to it. At least drink enough water.

It is only called proteinuria when the amount of excreted protein is high, above 300 mg per day. You would probable have other symptoms as well.

It is common to get foamy urine at times especially a level up to 30 mg per day is considered insignificant.

Microalbuminuria is the name for protein in the urine between 30-300 mg per day. This is commonly seen in people with high blood pressure, diabetes type 2 tendencies, or kidney stones.

Note:

The presence of albuminuria is a powerful predictor of renal and cardiovascular risk in patients with type 2 diabetes and hypertension. In addition, multiple studies have shown that decreasing the level of albuminuria reduces the risk of adverse renal and cardiovascular outcomes. The pathophysiology is not definitively known, but is hypothesized to be related to endothelial dysfunction, inflammation, or possibly abnormalities in the renin-angiotensin-aldosterone system.

Read more here:

http://care.diabetesjournals.org/content/31/Supplement_2/S194.full