It's happened in some notable cases; for example, a major review of reboxetine, a Pfizer antidepressant, obtained unpublished data from German health regulators

It's happened in some notable cases; for example, a major review of reboxetine, a Pfizer antidepressant, obtained unpublished data from German health regulators and demonstrated that there was no benefit over placebo. The published data gave the opposite impression. Eyding, D., Lelgemann, M., Grouven, U., Härter, M., Kromp, M., Kaiser, T., et al. (2010). Reboxetine for acute treatment of major depression: systematic review and meta-analysis of published and unpublished placebo and selective serotonin reuptake inhibitor controlled trials. BMJ, 341. doi:10.1136/bmj.c4737 But yes, it's not standard practice. Meta-analyses try to check for publication bias but they do not systematically hunt down unpublished results.